Superconducting magnets are typically used in a variety of contexts, including nuclear magnetic resonance (NMR) analysis and magnetic resonance imaging (MRI). To realize superconductivity, the magnet is maintained in a cryogenic environment at a temperature near absolute zero. Typically, the magnet comprises an electrically conductive coil which is disposed in a cryostat containing a volume of a cryogenic fluid such as liquid helium. Many such superconducting magnets operate in “persistent mode.” A magnet operating in persistent mode is initially energized with current from an external power supply to start up its magnetic field. The power supply is then disconnected from the magnet. The current and the magnetic field are maintained due to the magnet's superconductivity.
Although a continuous supply of power is not required to sustain the magnetic field, power (e.g., AC Mains power) are typically supplied to a compressor, which drives a cooling unit or “cold head” that maintains the temperature in the cryostat near absolute zero so that the magnet's superconductivity can persist.
In the event of a power loss to the compressor, the cold head may cease to operate and conditions within the cryostat can degrade, i.e., the temperature of the magnet can begin to rise. If power is not reapplied to restore cooling of the magnet's environment, the magnet's temperature may rise to a critical temperature where the magnet will “quench” and convert its magnetic energy to heat energy, thereby heating the cryogenic fluid within the cryostat. This “quench” can cause some or all of the cryogenic fluid to evaporate and be lost, for example, through a pressure relieve valve. Furthermore, the heat may damage the magnet and/or other components of the system.
Once power is restored, returning the magnet to superconducting operation may require: replacing the lost cryogenic fluid within the cryostat, cooling the magnet to below the critical temperature, and regenerating the magnetic field. Furthermore, if heat from the quench caused the magnet or other components to be damaged, they may need to be repaired or replaced.
This recovery process can be expensive and time-consuming. Typically, a trained technician must be dispatched to the facility (e.g., a medical center or hospital) where the superconducting magnet system is located and new cryogenic fluid (e.g. liquid helium), which may be quite costly, must be supplied to the cryostat.
Nevertheless, MRI systems typically employ relatively large volumes of cryogenic fluid (e.g., 1000 liters of liquid helium), which can at least partially ameliorate such a situation. This large volume of cryogenic fluid has a large thermal mass which can prevent the magnet from reaching the critical temperature for extend periods—possibly even days. Furthermore, such a superconducting magnet system typically provides access by which a user may add cryogenic fluid to the cryostat to replace lost or evaporated cryogenic material.
However, some newer MRI systems employ so-called “cryofree” superconducting magnet systems which are closed or sealed and do not include any means for a user to add new cryogenic material to the system. Furthermore, such closed systems typically have a smaller volume of cryogenic material when compared with conventional systems as described above (e.g., one liter of liquid helium). Accordingly, quench may occur in a relatively short amount of time (e.g., several hours) after a power failure to the compressor. Furthermore, since the systems typically do not allow additional cryogens to be added, if the cryogenic liquid is degraded or evaporated due to a quench, recovery may not be possible.
In one exemplary embodiment of the present invention, an apparatus can be provided, comprising, for example: an electrically conductive coil configured to produce a magnetic field when an electrical current is passed therethrough; a persistent current switch connected across the electrically conductive coil and configured to be selectively activated and deactivated; a cryostat having the electrically conductive coil and the persistent current switch disposed therein; an energy dump unit; at least one sensor configured to detect an operating parameter of the apparatus and to output at least one sensor signal in response thereto; and a magnet controller configured to receive the at least one sensor signal and in response thereto to detect whether an operating fault exists in the apparatus, and when the operating fault is detected, to connect the energy dump unit across the electrically conductive coil by the first and second electrically conductive leads so as to transfer energy from the electrically conductive coil to the energy dump unit, which disperse the energy outside of the cryostat.
In some embodiments, the energy dump unit is disposed outside of the cryostat and is physically connected to the cryostat so as to transfer heat from the energy dump unit to the cryostat.
In some embodiments, the apparatus further comprises at least one cooling fan which is supplied by a voltage across the energy dump unit and which is activated when the energy dump unit is connected across the electrically conductive coil in response to the detection of the operating fault.
In some embodiments, the apparatus further comprises first and second electrically conductive leads which are retractable and extendable under control of the magnet controller, and are configured in a retracted position to be disposed substantially entirely outside the cryostat and in an extended position to extend into the cryostat; and third and fourth electrically conductive leads disposed within the cryostat and connected to opposite ends of the electrically conductive coil. The magnet controller is configured to extend the first and second electrically conductive leads to be engaged with, and electrically connected to, the third and fourth electrically conductive leads, respectively, in response to detecting that an operating fault exists in the apparatus, and the first and second electrically conductive leads are connected to the energy dump unit.
In some embodiments, the apparatus further comprises: a cryogenic heat shield disposed within the cryostat; first and second electrically conductive leads; and third and fourth electrically conductive leads disposed within the cryostat and connected to opposite ends of the electrically conductive coil. Each of the third and fourth electrically conductive leads comprises a material which is superconducting at a temperature above 50° K. The third and fourth electrically conductive leads are anchored thermally to the cryogenic heat shield. The first and second electrically conductive leads is configured to be connected to the third and fourth electrically conductive leads, respectively, under control of the magnet controller in response to detecting that an operating fault exists in the apparatus, and the first and second electrically conductive leads are connected to the energy dump unit.
In some embodiments, the apparatus further comprises a switch configured to selectively connect one of the first and second electrically conductive leads to the energy dump unit in response to a control signal from the magnet controller
In some embodiments, the apparatus further comprises first and second electrically conductive leads configured to connect the energy dump unit across the electrically conductive coil, and the first and second electrically conductive leads have first ends permanently disposed inside of the cryostat, and further have second ends permanently disposed outside of the cryostat.
In some embodiments, the apparatus further comprises: a cold head configured to cool the cryostat; a compressor configured to drive the cold head; an inner chamber within the cryostat; and a thermal insulation region disposed between an outer wall of the cryostat and the inner chamber. The sensor(s) includes at least one of: a first temperature sensor configured to measure a temperature of the electrically conductive coil, a second temperature sensor configured to measure a temperature of the cold head, a third temperature sensor configured to measure a temperature in the thermal insulation region, a sensor configured to monitor a level of cryogenic fluid within the cryostat, and a sensor configured to determine if the compressor is properly driving the cold head.
In some embodiments, the apparatus further comprises: a cold head configured to cool the cryostat; a compressor configured to drive the cold head; an inner chamber within the cryostat; and a thermal insulation region disposed between an outer wall of the cryostat and the inner chamber. The sensor(s) includes: a first temperature sensor configured to measure a temperature of the electrically conductive coil, a second temperature sensor configured to measure a temperature of the cold head, a third temperature sensor configured to measure a temperature in the thermal insulation region, a sensor configured to monitor a level of cryogenic fluid within the cryostat, and a sensor configured to determine if the compressor is properly driving the cold head.
In some embodiments, the apparatus further comprises: a cold head configured to cool the cryostat; a compressor configured to drive the cold head, the compressor configured to receive power from AC Mains; and a backup power supply configured to supply power to the magnet controller when AC Mains incurs a power outage.
In some embodiments, the apparatus is a magnetic resonance imaging (MRI) apparatus. The MRI apparatus further comprises: a patient table configured to hold a patient; gradient coils configured to at least partially surround a portion of a patient for which the MRI apparatus generates an image; a radio frequency coil configured to apply a radio frequency signal to the portion of a patient and to alter the alignment of this magnetic field; and a scanner configured to detect changes in the magnetic field caused by the radio frequency signal.
In some embodiments, the apparatus further comprises: a closed system within the cryostat having a cryogenic fluid disposed therein; and a cold head configured to cool the cryogenic fluid within the closed system.
In another aspect of the invention, a method can be provided for operating a magnet system including an electrically conductive coil disposed in a cryostat. The method can comprise, for example: generating a persistent magnetic field by the electrically conductive coil; monitoring at least one sensor signal produced by at least one sensor in the magnet system; and in response to the at least one sensor signal, detecting by a magnet processor whether an operating fault exists in the magnet system. When the operating fault is detected, an energy dump unit is automatically connected across the electrically conductive coil so as to transfer energy from the electrically conductive coil to the energy dump unit, and a heater in the persistent current switch connected across the electrically conductive coil is activated.
In some embodiments, the sensor signal(s) is produced by at least one of: a first temperature sensor configured to measure a temperature of the electrically conductive coil, a second temperature sensor configured to measure a temperature of a cold head configured to cool the cryostat, a third temperature sensor configured to measure a temperature in a thermal insulation region between an inner chamber of the cryostat and an outer wall of the cryostat, a sensor configured to monitor a level of cryogenic fluid within the cryostat, and a sensor configured to determine if a compressor configured to drive the cold head is properly driving the cold head.
In some embodiments, the sensor signal(s) is produced by: a first temperature sensor configured to measure a temperature of the electrically conductive coil, a second temperature sensor configured to measure a temperature of a cold head configured to cool the cryostat, a third temperature sensor configured to measure a temperature in a thermal insulation region between an inner chamber of the cryostat and an outer wall of the cryostat, a sensor configured to monitor a level of cryogenic fluid within the cryostat, and a sensor configured to determine if a compressor configured to drive the cold head is properly driving the cold head.
In some embodiments, the magnet system can include first and second electrically conductive leads which are retractable and extendable, each of the first and second electrically conductive leads being configured in a retracted position to be disposed substantially entirely outside the cryostat, and in an extended position to extend into the cryostat, and third and fourth electrically conductive leads, respectively, which are disposed within the cryostat and which are connected to opposite ends of the electrically conductive coil. Automatically connecting the energy dump unit across the electrically conductive coil can include extending the first and second electrically conductive leads to be engaged with and electrically connected to, the third and fourth electrically conductive leads.
In some embodiments, the magnet system can include first and second electrically conductive leads which have first ends disposed inside of the cryostat and connected to opposite ends of the electrically conductive coil, and further have second ends disposed outside of the cryostat, and automatically connecting the energy dump unit across the electrically conductive coil comprises selectively connecting one of the first and second electrically conductive leads to the energy dump unit in response to a control signal from the magnet controller.
In yet another aspect of the invention, a magnet system is provided. The magnet system comprises: an electrically conductive coil configured to produce a magnetic field when an electrical current is passed therethrough; a persistent current switch connected across the electrically conductive coil and configured to be selectively activated and deactivated; a cryostat having the electrically conductive coil and the persistent current switch disposed therein; an energy dump unit; at least one sensor configured to detect an operating parameter of the apparatus and to output at least one sensor signal in response thereto; and a magnet controller The magnet controller is configured to monitor the at least one sensor signal, and in response to the at least one sensor signal, detect whether an operating fault exists in the magnet system. When the operating fault is detected, the energy dump unit is automatically connected across the electrically conductive coil so as to transfer energy from the electrically conductive coil to the energy dump unit, and a heater in the persistent current switch is activated.